In modern wind turbines the rotor blades are mounted to the hub via pitch bearings which allow for controlling the loads acting on the blades by changing the pitch of the blades relative to the wind. Those bearings usually comprise a cylindrical inner bearing ring to which the rotor blade is fixed and a cylindrical outer bearing ring which is fixed to the rotor hub. Between both bearing rings one or more rows of rolling elements are present which allow the outer ring and the inner ring to be rotated relative to each other.
When bearings with more than one row of rolling elements are used the load can become unevenly distributed between the rows, especially in strong winds when the wind forces act to substantially bend the blades. This bending also has the effect of deforming the outer ring of the pitch bearing in such a way that the load on some of the rolling elements is further reduced leaving the rest of the rolling elements to carry a larger load. Hence, parts of the bearing carry a larger load than intended, leading to a shortened lifetime compared to the designed lifetime.
In some of the blade bearing designs the uneven sharing between the rows are not taken into account. Instead it is assumed that the sharing is exactly evenly distributed which leads to a non-conservative design. One way to solve the issue of unevenly distributed loads would be to make the bearing larger. However, this would add weight and cost to the design.
Another way to obtain more load bearing capacity would be using larger bearing balls. The drawback of using larger bearing balls would be that this will require a higher bearing and thereby a more flexible bearing. As a consequence, it will become more difficult to obtain the required stiffness of the bearing.
A further way of resolving the issue of unevenly distributed loads is described in WO 2007/003866 A1. In this document it is described to mount a reinforcement ring to the axial outer end of the cylindrical outer bearing ring.